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Evolving threat to tankers in Persian Gulf
Iranian forces have long emphasized asymmetrical tactics in the Straits of Hormuz. The seizure of the British flagged tanker Stena Impero (IMO: 9797400) on 19th July 2019, and boarding of the MV Mesda (IMO: 9452672) the same day, serve as a reminder of the ease with which Iran’s Islamic Revolutionary Guards Naval forces (IRGC-N) can approach tankers and other merchant ships in the Straits.

Although the boarding was conducted by fast-roping from a Mi-8 HIP helicopter, the tanker was first approached by multiple IRGC-N fast boats which placed themselves behind and on the port flank, i.e. the opposite side to the Iranian coast]:

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Boarding is the least of tanker’s worries however. The seizure of Stena Impero is a specific UK-Iran topic motivated by Iran’s desire to retaliate for the seizure of Grace 1 in Gibraltar. The greater threat comes from limpet mines, and Iran has developed new tactics to employ them against moving targets.

The 13th June attacks on two tankers near the Persian Gulf represent a new threat to shipping in these globally important shipping lanes. Two limpet mines were placed on each vessel (Front Altair and Kokuka Courageous) from a small boat. Limpet mines, by both tradition and design, are supposed to be attached below the waterline to maximize destruction. They use powerful magnets to cling to the ship's hull below the waterline and punch a hole in the steel plate and buckle internal bulkheads causing flooding, and ultimately sinking.
Iranian Covert Operations ship Saviz - Covert Shores

Modern tankers, by virtue of their size and construction, are not particularly vulnerable to diver-carried limpet mines however. There is only so much explosives that a diver can swim with. The attacks on four tankers at Fujairah on 12th involved below-waterline placement of the mines, although underwater drones have also been suggested. Damage was modest.
Iranian Covert Operations ship Saviz - Covert Shores

Placing a limpet mine beneath the waterline on a moving target isn't a viable tactic because of the flow of water. So the new tactic is improvised. It would however leave less evidence than rocket, torpedo or larger mine attacks. When placed above the waterline they are unlikely to cause serious flooding, but fire is a possibility as the Front Altair shows.

Iran's Revolutionary Guard (IRGC) routinely inspects international shipping movements from speedboats, frequently getting close enough to read the name. This practice verifies radar, radio and automatic identification system (AIS) credentials broadcasted by the ships, and sends the clear message that Iran could strangle the world's oil supply at this vulnerable choke point. There is very little that a civilian tanker can do to prevent contact, even when the Threat Level is at critical. A month after the above-waterline limpet mine attacks the Royal Navy frigate HMS Montrose used tactics to deter Iranian boats getting close to the British tanker British Heritage (IMO: 9682980). The July 19th incidents demonstrate that there are not enough escort vessels in the Straits to meet every approach.

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These inspections are routine and could provide cover for placing the mines.

The attacks went wrong however. One of the mines on failed to explode. An Iranian patrol boat later approached the tanker and removed it. Limpet mine fuses are often unreliable but mines typical have a reserve fuse designed to detonate it when another nearby mine explodes. This is called a sympathetic explosion . So ordinarily the first mine should have set off the second one. Possibly because the mines were above the waterline this didn't happen. Whatever the reason it left a smoking gun which helps navies and ship operators better assess how it likely happened, and potential tactics to defend it happening again.
Iranian Covert Operations ship Saviz - Covert Shores
Iranian Covert Operations ship Saviz - Covert Shores

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